Use the original Themis files again, and find this in index.template.php:
echo '
</head>
<body style="margin-left: 20px; margin-right: 20px;"><table width="100%" cellpadding="0" cellspacing="0">';
Replace with this (change 800 to your preference) :
echo '
</head>
<body style="margin-left: 20px; margin-right: 20px;"><table width="800" cellpadding="0" cellspacing="0" align="center">';
just a note...if you use the sidebar, it will be included in the total width, making the forum even smaller. So if you want it to stay centered and same width, just dont use the sidebar at all.